I was wondering because the info page on AA said "The prototype was ultimately sold to an AtariAge member, who sent us the game so we could dump it as well as take pictures and scans of the prototype board and cartridge.

There is a thread located here, where it was said last July that the game is still being worked on. Apparently there are plans to release it on cart when it's completed. It's been in the works for a few years, so it'd be interesting to have an idea of how much longer the wait will be.

Frankly, you're not missing out on that much. The proto is pretty much umplayable.
The 'enhanced' version included with the release is playable, though frankly I think Enduro and Pole Position are both more fun to play.

As for why the ROM hasn't been released, my guess is Albert hasn't sold enough copies of the proto to recoup expenses yet. There's a thread around here somewhere to that effect, but I'm entirely too lazy to search for it.